Abstract:
Samples from 14 spinel occurrences in the Lake Harbour Group (LHG), Baffin Island, Nunavut, Canada, were studied using a combination of petrography, whole rock geochemistry, microprobe analysis, and, where possible, geochronology. Spinel at most occurrences is blue to violet and generally not of gem quality. Two spinel occurrences near Kimmirut contain vivid blue, cobalt-enriched (0.03–0.07 wt.% CoO) spinel. The spinel mostly occurs in metasedimentary (sensu stricto) rocks, with the exception of a few metasomatic occurrences. All spinel occurs in marble and calc-silicate/silicate-rich metacarbonate rocks. Minerals occurring with spinel as part of a stable assemblage include calcite, dolomite, phlogopite, pargasite, diopside, humite, forsterite, scapolite, anorthite, graphite, and pyrrhotite. The spinel formed under peak granulite facies metamorphic conditions.
Metasedimentary (s.s.) spinel-bearing rocks in the LHG are interpreted to have the following protoliths: (1) impure dolomite-bearing and dolomitic limestone; (2) dolomitic marl; and (3) evaporitic magnesitic marl. Other significant gemstone occurrences in the LHG (sapphire and lapis lazuli) are interpreted to have similar protoliths. Factors that favor spinel genesis in Mg-bearing metacarbonates include: (1) a low abundance of Si relative to Al, which is a primary control on whether spinel forms in most calc-silicate rocks; (2) low K activity limiting the formation of phlogopite and thus leaving Al available for spinel formation; (3) low XCO2 in marbles; and (4) insufficient quantities of Mg or dolomite reactant in diopsidite limiting Al incorporation into phlogopite to form spinel.
The spatial distribution of Co enrichment at the cobalt-blue spinel occurrences is indicative of highly localized enrichment. Cobalt and Ni are interpreted to have been enriched in the original sediment, or during diagenesis or low-grade metamorphism. Concentrations of Co (up to 29 μg/g) and Ni are anomalously high, while concentrations of Fe, Mn, V, Cr, and Cu are much lower than expected; this chemical signature speculatively could be caused by diagenetic processes prior to metamorphism.
Pyrrhotite strongly partitions Fe relative to spinel, and therefore an abundance of sulfide is expected to improve the attractiveness (and commercial value) of spinel by decreasing the amount of Fe incorporated, thus preventing spinel from having an overly dark color.

Marco E. Ciriotti

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